The present invention relates to reactors in which the cooling of the core is brought about by the continuous circulation of a liquid metal, namely sodium, contained within a main vessel in which is immersed the core, the calories acquired by the liquid metal on passing through the latter on contact with the fuel assemblies being transferred to a secondary fluid, which is also sodium, which internally passing through the tubes of an intermediate exchanger which pass into the vessel, the liquid cooling metal of the core acting as the primary fluid circulating externally of said tubes in thermal exchange relationship with the secondary fluid. In known manner, the secondary fluid which is in this way heated is then returned externally of the reactor vessel to a generator able to supply pressurized steam directly expanded in an electricity generation plant.
The present invention more specifically relates to an arrangement of the internal structure of such an intermediate exchanger comprising in per se known manner a vertically axed cylindrical internal ferrule, an external ferrule coaxial to the internal ferrule, two plates with annular horizontal tubes located in the vicinity of the upper and lower ends of said ferrules, a bundle of straight tubes extending between the tubed plates in the form of cylindrical layers coaxial to the ferrules, said layers being mutually reinforced by transverse belts formed by horizontal bands carrying spacing members located between the tubes of the layers and in contact with the latter, collectors for theadmission and discharge of the secondary fluid circulating within said tubes being respectively provided beneath the lower plate and above the upper plate, the inner ferrule forming a pipe for the supply of the secondary fluid to the admission collector, whilst the other ferrule is upwardly extended by a pipe for the discharge of the same secondary fluid collected in the discharge collector after passing through the tubes, and inlet and outlet ports distributed about the axis of the outer ferrule for the admission and discharge, in the vicinity of the tubed plates, of a primary fluid which exchanges calories with the secondary fluid through the wall of the tubes.
In a conventional construction of this type, the primary fluid therefore penetrates in a transverse manner the exchanger by the inlet windows, being distributed in the space between the inner and outer ferrules in order to circulate after a first change of direction of essentially 90.degree. in contact with the tubes in the bundle, the flow taking place over most of the length of said tubes in countercurrent to the secondary fluid, the primary fluid undergoing a second change of direction of once again 90.degree. and is then discharged from the exchanger by the outlet ports.
It is clear that as a result of this double change of direction, there is a significant variation in the temperature of the secondary fluid on leaving the tubes of the bundle in the discharge collector, the cylindrical layers located as close as possible to the inner ferrule permitting the discharge of a cooler secondary fluid then the tubes of the layers close to the outer ferrule, the heat exchange with the primary fluid having been less effective insaid latter inner layers. This variation is particularly due to a reduced supply of primary fluid to the tubes of the inner layers, particularly level with the inlet and outlet ports. As a result, there are significant differential expansions between the inner and outer ferrules, which leads to high mechanical stresses which are prejudicial to the good behaviour of the exchanger.